A commercially important type of magnet is an isotropic magnet. Isotropic magnets can comprise numerous alternating north and south poles, creating complex magnetic field patterns. The alternating north and south poles are associated with independent magnetic units (called domains) which are not initially magnetically aligned with each other. Such domains are optimally kept very small to increase the number of independent domains per unit area. As the crystal size, or grain size, of a magnetic material typically defines the maximum domain size of magnets formed from the material, it is advantageous to form the material into extremely fine grain sizes.
Isotropic magnets frequently comprise alloy mixtures of iron (Fe), neodymium (Nd), and boron (B), typically of the general formula Nd.sub.2 Fe.sub.14 B. The processing of alloys having a formula of about Nd.sub.2 Fe.sub.14 B is metallurgically complex and requires careful control to obtain a homogeneous distribution of elements necessary for good magnetic properties.
The fine grain size necessary for the single grain/single domain structure of isotropic magnets can only be obtained by rapid solidification of a molten alloy. Presently, two classes of processes are known which may be utilized for rapidly cooling an alloy mixture. The first class encompasses melt-spinning processes. In melt-spinning processes an alloy mixture is flowed onto a surface of a rapidly spinning wheel. Upon contacting the wheel surface, the alloy mixture spreads into a flake-like powder, typically having a size and texture of glitter. The rate of cooling of the mixture can be controlled by controlling the rate of spinning of the wheel. Typically, the wheel will be spun at a rate such that a wheel surface has a tangential speed of about 25 m/sec to achieve a cooling rate on the order of about 10.sup.6 .degree. C./sec.
The glitter-like flakes resulting from a melt-spinning process can be crushed into a powder and incorporated into an isotropic magnet. The majority of isotropic magnets are of an MQ1 type made by combining isotropic powders with epoxy and compression molding the epoxy/powder combination into a desired form. Higher strength (mechanical as well as magnetic) magnets can be made by hot-pressing isotropic powders into a fully dense (or MQ2) form. Such hot-pressing typically involves compressing and shaping a magnet powder at temperatures of 725.degree. C. or higher.
A cooling rate on the order of 10.sup.6 .degree. C./sec is required to obtain good-quality magnetic properties from Nd.sub.2 Fe.sub.14 B. This is illustrated in the graph of FIG. 1 which shows the relationship between the cooling rate of a melted alloy comprising Nd.sub.2 Fe.sub.14 B and a maximum energy product (BH.sub.max) of an alloy powder produced from the cooled alloy.
As shown in FIG. 1, if a cooling rate is too slow a low maximum energy product is obtained. A reason for the low maximum energy product is that the alloy mixture separates into different phases during the slow cooling. Thus, the slowly cooled alloy has a microstructure consisting of multiple phases, which is an inferior product. Also, the slow cooling can disadvantageously lead to formation of large crystals, creating unwanted large magnetic domains. The inferior products produced by too-slowly cooling the alloy mixture are referred to as "underquenched".
At another extreme, if the melted alloy is cooled too quickly it forms an amorphous glass which also has an inferior maximum energy product. The inferior products produced by too-quickly cooling the alloy mixture are referred to as "overquenched".
Between the two extremes of overquenching and underquenching a melted alloy is an optimal cooling rate which creates an alloy powder having a peak maximum energy product. A peak maximum energy product is obtained if the melted alloy cools at a rate sufficient to form a nanocrystalline alloy powder.
Generally, it is commercially impractical to obtain a cooling rate precisely capable of forming a powder at its peak maximum energy product. Accordingly, the melted alloy is typically slightly overquenched to form an alloy powder which comprises amorphous and nanocrystalline internal structures. Subsequently, the overquenched material is heat treated. Such heat treatment converts the amorphous structure of the alloy mixture to a microcrystalline phase and thus converts the alloy powder to a form having approximately a peak maximum energy product. The heat treatment typically comprises heating the alloy powder to a temperature of less than or equal to about 650.degree. C. for a time sufficient to improve magnetic properties, such as for example, about four minutes.
Currently, the melt-spinning process is the only commercially available process known which can achieve the necessary rapid cooling rates of 10.sup.6 .degree. C./sec to form good quality magnetic powders from Nd.sub.2 Fe.sub.14 B. Thus, the melt-spinning process is the only commercially feasible process for producing a powder for an isotropic magnet.
The second class of processes are atomization processes. Atomization processes have potential for forming isotropic magnet powders, but are currently in very limited commercial use. The magnet powders produced by atomization processes differ from those produced by melt-spinning processes in that a magnet powder formed from an atomization process is comprised of generally spherical alloy powder granules, whereas those produced by a melt-spinning process are comprised of flake structures. Atomization processes include water atomization, vacuum atomization, centrifugal atomization, and gas atomization processes.
An example atomization process is a gas atomization process. Gas atomization of rare earth permanent magnets has been investigated for over a decade. Gas atomization potentially offers an advantage over melt-spinning in that a gas atomization apparatus can produce a magnet powder at a rate of tons per hour, whereas a melt-spinning apparatus only produces a magnet powder at a rate of about 100 pounds per hour. A disadvantage of gas atomization processes is that the cooling rate of such processes is typically 10.sup.5 .degree. C./sec or less, which results in an underquenched Nd.sub.2 Fe.sub.14 B.
A gas atomization apparatus 10 is illustrated in FIG. 2. Apparatus 10 comprises a melting chamber 11, a drop tube 12 beneath melting chamber 11, a powder collection chamber 14, and a gas exhaust 16.
Melting chamber 11 includes an induction melting furnace 18 and a vertically movable stopper rod 20 for controlling a flow of a melt from furnace 18 to a melt atomizing nozzle 22 between furnace 18 and drop tube 12. Atomizing nozzle 22 is supplied with an inert atomizing gas (for example, argon or helium) from a suitable source 24. Source 24 can be a conventional bottle or cylinder of the appropriate gas. Atomizing nozzle 22 preferably atomizes the melt into the form of a spray of generally spherical molten droplets discharged into drop tube 12. The droplets solidify as they fall through discharge tube 12 to form a powder which accumulates in powder collection chamber 14. The powder generally has the consistency of flour.
Melting chamber 11 and drop tube 12 can be connected to an evacuation device (for example, a vacuum pump) 30 via suitable ports 32, conduits 33 and valves 34.
Drop tube 12 is generally filled with a room temperature gas. However, drop tube 12 can also be filled with a liquid gas for more rapid cooling.
A general disadvantage of atomization processes is that the processes typically only cool at a rate of about 100,000.degree. C./sec. Such a cooling rate is too slow to form the slightly overquenched Nd.sub.2 Fe.sub.14 B-comprising powder preferred in commercial processes. Thus, although atomization processes, such as, for example, gas atomization, are recognized as having potential advantages over melt-spinning processes, atomization processes are generally not used commercially for forming magnet powders.
Several attempts have been made to improve atomization processes to the point that they are commercially feasible. Among such attempts have been efforts to form alloy mixtures with cooling properties suitable for the relatively low-cooling-rate atomization process. Instead of Nd.sub.2 Fe.sub.14 B, alloy mixtures having a significantly higher rare-earth content and a significantly lower iron content are utilized for atomization processes. The use of alloy mixtures having relatively high ratios of rare earth elements to other elements favorably changes the cooling properties of the alloy mixture so that the mixture can form powders having good magnetic properties under the relatively low-cooling-rate conditions of atomization processes. Unfortunately, the high ratios of rare earth elements also create undesired properties of increased corrosion relative to the Nd.sub.2 Fe.sub.14 B utilized in melt-spin processes, and decreased magnetic properties due to a lower volume of the Nd.sub.2 Fe.sub.14 B phase relative to the alloy utilized in melt-spin processes. The increased corrosion is due to the presence of the additional rare earth elements, which oxidize rapidly at room temperature, and which may even spontaneously erupt into flame at room temperature. The rare earth elements tend to corrode particularly rapidly at temperatures above 150.degree. C. The decreased magnetic properties are due to a decrease in the relative amount of iron in the total alloy mixture.
The increased corrosion of the rare earth rich alloy mixtures can become particularly problematic during hot-pressing processes of magnet formation which, as discussed above, typically involve heating a magnet powder to temperatures of 725.degree. C. or higher. Another drawback of the rare earth rich alloy mixtures relative to the Nd.sub.2 Fe.sub.14 B alloys utilized in melt-spinning processes is that the decreased magnetic properties of the rare earth rich alloy mixtures can be worsened during bonded magnet formation as the alloy is diluted with epoxy. For these reasons magnet powders comprising the rare earth rich alloy mixtures utilized in atomization processes are less preferred for use in magnet forming processes then are magnet powders comprising the Nd.sub.2 Fe.sub.14 B alloy mixes utilized by melt-spinning processes. Accordingly, commercial processes are melt-spinning processes, even though, as discussed above, there would be significant advantages in production capacity if an atomization process, such as, for example, a gas atomization process, were commercialized.
Recently, it has been found that the addition of titanium and carbon to an alloy mixture of Nd.sub.2 Fe.sub.14 B will alter the cooling properties of the alloy mixture. Methods for utilizing titanium and carbon to alter the cooling properties of an Nd.sub.2 Fe.sub.14 B alloy mixture are described in U.S. Pat. No. 5,486,240 to McCallum, et al., which issued on Jan. 23, 1996, and which is incorporated herein by reference. McCallum, et al. applied the methodology of titanium and carbon incorporation toward melt-spinning processes. It would be desirable to develop new alloy mixtures for adjusting the cooling rate of atomization processes.
An additional disadvantage of atomization processes can be that they are difficult and expensive to run at even a lab-scale. Accordingly, it would be desirable to develop methods for testing atomization processes which do not require running atomization processes at a lab-scale.